The inside of a lung looks nothing like a hollow balloon. If you could slice one open, you’d see a dense, spongy mass of tissue in shades of pink, riddled with thousands of tiny holes and branching tubes that resemble a tree growing upside down. It’s wet, springy to the touch, and far more intricate than most people expect.
The Branching Airway Tree
The most striking feature inside a lung is the bronchial tree, a system of airways that divides again and again like branches splitting from a trunk. Starting from the windpipe, each airway forks into two smaller passages. This happens roughly 23 times, producing tubes that range from the width of your thumb down to passages thinner than a strand of hair. The first 16 or 17 levels of branching are purely for moving air. They don’t absorb any oxygen themselves. Their walls are reinforced with rings and plates of cartilage that keep them propped open, and they’re lined with a glistening layer of mucus that traps dust and debris before it reaches deeper tissue.
As the branches get smaller, the cartilage gradually disappears. The tiniest airways, called bronchioles, rely on the surrounding spongy tissue to hold their shape. If you were to look at a cross-section of the lung under a magnifying glass at this level, you’d see these narrow tubes embedded in a dense web of soft, elastic tissue, almost like tunnels running through a sponge.
The Spongy Tissue: Millions of Air Sacs
At the very end of those 23 branching levels sit clusters of tiny air sacs called alveoli. This is what gives the lung its spongy texture. Each alveolus is a hollow pocket roughly 0.2 millimeters across, smaller than a grain of salt. Your lungs contain about 150 million of them. Under a microscope, a slice of healthy lung tissue looks like a honeycomb or a sheet of bubble wrap, with thin walls separating one air pocket from the next.
Those walls are extraordinarily thin. Each one is densely packed with a net of the tiniest blood vessels in the body, capillaries so narrow that red blood cells pass through them in single file. The barrier between the air inside an alveolus and the blood flowing past it is less than a millionth of a meter thick. Oxygen slips across this barrier into the blood, and carbon dioxide moves the other direction, all without any pumping or active effort. It happens purely because gases naturally move from areas of high concentration to low.
If you could somehow unfold every one of those 150 million air sacs and lay them flat, the total surface area would be roughly the size of a tennis court. All of that is folded and compressed into two organs that fit inside your ribcage. That enormous surface area is the reason lungs are so efficient at pulling oxygen from the air.
What Keeps the Air Sacs Open
The inside of each alveolus is coated with a thin film of fluid that contains a substance called surfactant, produced by specialized cells in the alveolar walls. Without surfactant, the surface tension of the moisture lining each tiny sac would cause it to collapse like a wet plastic bag sticking to itself. Surfactant lowers that surface tension, keeping the air sacs inflated and springy even when you breathe out. It also plays a role in the lung’s immune defenses, helping to calm inflammation and fight off inhaled pathogens.
The cells that make up the alveolar walls come in two main types. One type is flat and incredibly thin, stretched wide to maximize the area available for gas exchange. The other type is smaller and cube-shaped, and these are the cells responsible for producing surfactant. Together, they maintain the delicate architecture that makes the lung look and function like living bubble wrap.
Color and Surface Appearance
A healthy lung is pink. The color comes from the rich blood supply running through those capillary networks, much the same way the inside of your lip looks pink. The tissue is moist throughout, and the surfaces of the airways glisten with a thin layer of mucus. Tiny hair-like structures called cilia line the larger airways, beating in coordinated waves to sweep mucus (along with any trapped particles) upward and out of the lungs.
The outer surface of each lung is wrapped in a smooth, shiny membrane called the pleura. There are actually two layers: one hugging the lung surface and one lining the inside of the chest wall. Between them sits a thin film of clear, slightly yellowish fluid. This fluid acts as a lubricant, letting the lungs glide against the chest wall with every breath without friction.
How a Damaged Lung Looks Different
The contrast between a healthy lung and one that has been exposed to years of smoke or pollution is dramatic. Healthy lung tissue is uniformly pink, soft, and elastic. A smoker’s lung often appears gray or black, stained by carbon particles and tar that become permanently trapped in the tissue. The discoloration isn’t just surface-level. Cut into a long-term smoker’s lung and the black deposits run deep throughout the spongy tissue.
The internal changes go beyond color. Smoking damages the cells lining the airways, triggering inflammation that causes the tissue to swell and the airways to narrow. The mucus-producing cells multiply in response to the constant irritation, so the airways become coated with thicker, more abundant mucus. At the same time, the cilia that normally clear that mucus are paralyzed or destroyed, leaving the debris with no way out. Deeper in the lung, the walls between alveoli break down and merge, creating fewer, larger air pockets. This reduces the total surface area available for gas exchange and is the hallmark of emphysema. The capillary walls also thicken and scar, making it harder for oxygen to pass into the blood. A smoker’s lung often appears hyperinflated, larger than normal, because the damaged tissue loses its elastic recoil and traps air inside.
Even in people who have never smoked, lungs gradually accumulate some dark speckling over a lifetime from inhaling everyday dust, pollution, and fine particles. A child’s lung is a brighter, more uniform pink than an adult’s, simply because it has had less time to collect these deposits.